The present invention relates to the installation of sleeves in heat exchanger tubes for the purpose of repairing the heat exchanger tubes and, more particularly, to a new and useful method of explosively welding a sleeve into a heat exchanger tube at a location spaced away from a tubesheet or tube support plate that supports the heat exchanger tube.
Many heat exchangers, such as nuclear steam generators, employing tubes arranged in bundles by tube support plates and tubesheets, experience corrosive or mechanical damage in the vicinity of these structures which require repair. One method of making such a repair involves placing an internal sleeve inside the damaged tube at the location of the defect and with a length sufficient to extend beyond the defect. As a result, the sleeve often must extend beyond the supporting portions of the tubesheet or tube support plate. Further, to hold the sleeve in place and to prevent leakage around the sleeve, the sleeve must be sealed to the inner surface of the tube. Present methods of sealing the sleeve include expansion of the sleeve into the tube wall, brazing the sleeve to the tube wall, or explosive welding. The last mentioned technique, however, generally requires some sort of external support to prevent excessive deformation of the tube.
Explosive expansion or welding of tubes in heat exchangers for either installation or repair is well known in art. For example, U.S. Pat. No. 3,411,198, discloses plastic insert comprised of an explosive charge placed within an annular tubular force transmitting member made of polyethylene, having an air gap at one end thereof, for expanding a tube into a tubesheet.
U.S. Pat. No. 3,590,877 discloses an explosive activated metal cylindrical plug used to plug a defective heat exchanger tube in the vicinity of the externally supporting tubesheet. In this patent, shaped explosive charges are formed by wrapping sheets of the explosive around frustoconical and cylindrical styrofoam filler pieces, separated by a central cylindrical block of explosive.
U.S. Pat. No. 3,724,062 also discloses an explosive plug for a heat exchanger tube in the vicinity of a tubesheet. The plug has a central cylindrical section and end frustoconical sections with the explosive charge being cylindrically shaped and located in the central cylindrical section.
U.S. Pat. No. 3,790,060 discloses the use of a pair of explosive members, one of which serves to weld a pipe to the perforated plate through which it passes, while the second expands the pipe into bearing contact with the plate at flared or counterboard portions located at the faces of the perforated plate.
U.S. Pat. No. 3,912,148 discloses a combination welding and brazing device for repairing heat exchanger leaks at the junctions between the tubing and the associated tubesheets. A number of grooves filled with brazing metal are located on one end of the metal sleeve. The other end has a reduced thickness and accomodates a relatively large charge of nitroguanidine. A smaller expansion charge of PETN extends from the nitroguanidine through the center of the sleeve and past the grooves. When detonated, the thin portion of the sleeve (which is inserted into the tube sheet portion of the tube) is welded to the inner surface of the tube, while the other end that lies within the free-standing end is merely pressed into closer brazing contact with the tube inner surface. The actual brazing itself is accomplished in a second step by means of a cartridge type electrical resistance heater that is inserted into the sleeve/tube to heat and liquefy the brazing metal.
U.S. Pat. No. 3,912,148, which is assigned to the assignee of the present invention is of particular interest, however, in its description of the prior art existing at the time. The Patent indicates that explosive welding is suitable for use within the tubesheet and brazing is suitable for use within the free-standing portion of the tube bank, due to concerns of excessive tube deformation. The explosion used in the free-standing portion is used only to expand the brazing portion of the sleeve into close contact with the tube inner wall, not to accomplish the joining itself.
U.S. Pat. No. 4,028,789 also discloses a combination welding/brazing process, quite similar to U.S. Pat. No. 3,912,148, where the welding is done within the tubesheet supported portion of the tube and expansion is done in the free-standing portion of the tube.
U.S. Pat. No. 4,021,907 discloses an explosive plug for sealing a defective tube held in a heat exchanger tubesheet. Each end of the plug has a hollow cylindrical or frustoconical end filled with a similarly shaped explosive charge. The various embodiments of this patent show the use of a detonator on one end and a percussion pin on the other so that both charges are detonated almost simultaneously.
U.S. Pat. No. 4,513,903 discloses yet another sleeving apparatus and method for sleeving a defective tube within a tubesheet, where the sleeve is welded directly over the defective portion of the tube. U.S. Pat. No. 4,567,632, while disclosing an explosive method of repairing defective tubes outside of the tubesheet, employs an external support comprised of a low melting point alloy cast around the free-standing portions of the tubes. U.S. Pat. No. 4,587,904 discloses a debris free plug assembly for repairing a tube within a tubesheet.
In general, the aforementioned patents teach the explosive welding of a sleeve into a tube in an area surrounded by the tubesheet or tube support plates. Where an explosive charge is used to expand a sleeve or plug inside a heat exchanger tube, at a location which is not immediately within the tubesheet or tube support plate, explosive welding is not utilized. None of the noted patents disclose or suggest the possibility of explosively welding a sleeve having tapered ends, into the interior of a heat exchanger tube, at a location which is not mechanically supported from the outside.